Conventional semiconductor lighting elements are made in a standard rectangular profile. As the general semiconductor material has a refractive index greatly different from the package material, total reflective angle is smaller. Hence when the light generated by the semiconductor lighting elements reaches the critical surface with air the light greater than the critical angle generates total reflection and returns to the crystals on the lighting elements. In addition, the rectangle has four parallel cross sections. The probability of protons leaving the semiconductor at the interface is smaller. The protons can only be totally reflected until fully absorbed. The light is converted to heat. As a result, light generation effect is not desirable.
Changing light reflection is an effective way to improve light generation efficiency. The present approach is forming a recess 11 and a protrusive portion 12 at the surface of a substrate 10 to generate light scattering or diffraction on the lighting area (referring to FIG. 1) to increase external quantum efficiency and enhance light generating efficiency.
However, the structure of the recess 11 and protrusive portion 12 also makes manufacturing of the epitaxy in the follow-on process more difficult. It generally requires to control the epitaxy in a desired condition to get a cavity-free semiconductor layer to enhance light generating efficiency. But the epitaxy parameters such as temperature, pressure, gas flow, V/III flux ratio and dopant content affect transverse and lateral growing speed of the epitaxy. Referring to FIG. 2, while an Epi layer 20 starts to form epitaxy on a bottom surface 13 of the recess 11 and a flat surface 14 at an upper side of the protrusive portion 12 the lateral growing speed of the Epi layer 20 on the protrusive portion 12 is greater than the lateral growing speed of the recess 11, hence squeezing takes place between them and cavities 21 occur after the Epi layer 20 is formed (referring to FIG. 3). These cavities 21 cause light loss in the semiconductor lighting element and reduce internal quantum efficiency. As a result lighting efficiency and life span of the lighting element suffer.